A full answer to your question would be very difficult here but there is a reasonably straightfoward account in the Ethics of how one type of 'adequate knowledge' is possible. This account is not dogmatic in my view.
Three grades of knowledge
The three grades of knowledge are readiy set out. There is knowledge:
From signs; as for example when we hear or read certain words, we
recollect things and form certain ideas of them similar to them, through which
ideas we imagine things [Schol. Prop. 18, pt. 2]. These two ways of looking at
things I shall hereafter call knowledge of the first kind, opinion or imagination.
From our possessing common notions and adequate ideas of the proper-
ties of things [Corol. Prop. 38, Prop. 39, with Corol. and Prop. 40, pt. 2].
This I shall call reason and knowledge of the second kind.
Besides these two kinds of knowledge, there is a third, as I shall hereafter
show, which we shall call intuitive science [intuitive knowledge: GT]. This kind of knowing advances
from an adequate idea of the formal essence of certain attributes of God to the
adequate knowledge of the essence of things.
(Spencer Carr, 'Spinoza's Distinction Between Rational and Intuitive Knowledge', The Philosophical Review, Vol. 87, No. 2 (Apr., 1978), pp. 241-252: 242.)
Second grade of knowledge
The first grade of knowledge apppears a plausible candidiate, and might on its own rebut the charge of dogmatism. As regards the second grade of knowledge, Spinoza tells how we form common notions (notionum communium). Common notions are necessarily adequate and constitute knowledge albeit only of the second kind:
They are common notions are structural features
that all modes within an attribute share (E2P38; the corollary appeals to Lemma 2, which in turns follows from the definition of a body E2D1).
(Eric Schliesser, 'Spinoza on the Politics of Philosophical Understanding', Proceedings of the Aristotelian Society, New Series, Vol. 111 (2011), pp. 497-518
Published by: Oxford University Press on behalf of The Aristotelian Society: 512.)
In propositions 37-40 of Part II Spinoza explains that we can form
common notions from our perceptions of objects and that these notions
are necessarily adequate. His idea is, roughly, that all bodies [modes: GT], by virtue
of involving the conception of the same attribute, agree in certain respects; every body and every part of every body has something in
common. Spinoza's view of the relationship between mind and body
and his doctrine of perception lead him to hold that any such common
notion can only be perceived adequately. (At IIP13L2 Spinoza gives
"being capable of motion and rest" as an example of what is common
to all bodies.) The first scholium to proposition 40, the scholium just
preceding the classification, is concerned with distinguishing these
common notions from universal ideas drawn from experience which
we might confuse with common notions. Throughout, Spinoza is
talking about how we do and how we should handle our experience
of natural objects.
(Carr: 245.)
I omit conisideration of the third grade of knowledge, intuitive knowledge or scientia intuitiva, for two reasons. In the first place it raises difficulties which cannot be dealt with briefly; and secondly, the task was to counter the criticism that Spinoza's account of adequate knowledge is dogmatic. In presenting a case that the first and second grades of knowledge have argumentative support in Spinoza's text and are not merely assertively claimed, I believe I have vindicated him against the charge of dogmatism. This is not to say that Spinoza's arguments are correct as they stand. I do not need to show that the arguments are cogent, only that they are arguments which as such give the slip to the censure of dogmatism.
